Active emission energy field sensors find widespread use in many industrial, commercial, and consumer markets. One type of such a sensor employs the use of light emitters such as light emitting diodes to illuminate a sensing area; the light energy from such emitters is detected and processed to determine the introduction of objects into the sense field, or the motion of objects already in the sense field. Another type of sensor emits an electrostatic field, and detects fluctuations in the emitted field due to capacitive effects induced by objects introduced or moved in the sense field. Similar detectors are commercially available for magnetic, microwave, and acoustic field sensing.
Each type of sensor mentioned above has applicability to different sensing situations. For example, an active field magnetic sensor is receptive only to ferrous or other magnetic or paramagnetic materials, and finds application in situations where it is desirable to detect such materials only and not non-magnetic materials.
In my previous patent, "Optical Motion Sensor", U.S. Pat. No. 4,736,097, these sensor types are reviewed in light of their characteristics and disadvantages. In this previous patent, a technique is described that can be used to enhance the sensitivity, dynamic range, and discrimination ability of emitted field optical sensors and other sensors as well, by means of the generation of a balance or cancellation energy field. This cancellation field may be modulated in such a way as to null the detected energy field, thus preventing amplifier saturation, eliminating detector nonlinearities, and reducing the effects of stray energy fields such as ambient light in a light based detector.
In certain sensing applications, ambient light or stray energy fields are not a problem. For example, in fiber optic sensing using a bifurcated fiber bundle, the introduction of ambient light into the detector is rarely significant because of the narrow acceptance angle of the fiber itself. Unless the fiber is aimed directly at a light source, very little ambient light will reach the photodetector. Hence, the need for a mechanism to decrease or eliminate detector nonlinearities is reduced, permitting the potential reduction in cost of such a sensor. Other types of energy field sensor means are known to be purely linear, and thus do not suffer from nonlinear gain effects when stray fields are present. For example, inductive air-wound coils responsive to magnetic fields possess essentially pure linearity, and thus do not require an energy field balance mechanism. However, it is still generally desirable to provide a mechanism by which sensitivity and dynamic range may be increased in such applications.
The use of a cancellation energy field also requires the additional expense of the second energy emitter, which may also require an objectional amount of additional physical space. The second emitter may also be difficult to align consistently, although this is not a frequent problem. Also, some sensors may involve the use of non-time-varying energy fields, as may be the case with a non-pulsed light energy field. In such situations, a cancelling energy field cannot be used because there exists no AC signal component which may be cancelled. With such a sensor a DC coupled amplifier must be used, and the cancellation signal would need to be negative to provide cancellation, a requirement which is clearly impossible with a non-polar energy field such as light.
In many applications where my previous patent has been employed it has become clear that there is a need for intelligent control of the modulated cancellation feedback signal. For example, it has been found useful in many situations to provide a modulation of the feedback that is nonlinear with respect to the net detected signal amplitude, rather than a simple integration function as described in my previous application. For example, nonlinear feedback modulation can be used to produce a slow cancellation effect when the net signal is less than a set threshold level, and fast cancellation adjustment when the threshold is exceeded. In this particular case, a motion detector is formed with an ability to ignore signals arising from drift in the output of light emitter due to temperature effects, or from slowly changing backgrounds. A number of modulation schemes may be incorporated into a sensor, allowing a user of such a sensor to select among several such schemes to optimize performance and to vary the operating mode according to the application.
It has also been found in sensors using my previous invention that the thresholding of the detected signal could benefit from a greater degree of flexibility. For example, in some situations it is desirable to select a threshold scheme whereby only increases or decreases or both increases and decreases in net received signal result in a triggered condition. If only net increases are used to create a triggered condition, the resulting sensor will respond only to objects in the sense field that cause an increase in signal strength. If only net decreases are used to create a triggered state, then only object motion or activity causing a decline in signal will trigger the sensor's output. It may be desirable to select both modes simultaneously, making the sensor responsive to either event.
Another desired feature would be a high degree of long term stability coupled with enhanced operational flexibility, normally unattainable by analog processing. These features are attainable by the use of digital acquisition, signal processing, and control techniques.